Polymer substrates for radiation-induced graft polymerization and filter stock

ABSTRACT

A method for performing radiation-inducted graft polymerization on substrates in the form of webs of woven or non-woven fabric, which includes the steps of exposing a substrate woven or non-woven fabric composed of polymer fiber to electron beams in a nitrogen atmosphere, contacting the irradiated substrate with a specified amount of monomer in a nitrogen atmosphere, and subjecting the monomer and the substrate in mutual contact to graft polymerization in a nitrogen atmosphere, characterized in that the first through third steps are performed in succession.

TECHNICAL FIELD

[0001] This invention relates to polymer substrates forradiation-induced graft polymerization that are made from polymerfibers, particularly from polyethylene fiber, as well as radiation grafttreated stock that is produced by introducing functional radicals intothe substrates by means of radiation-induced graft polymerization. Theinvention also relates to an improvement in the method ofradiation-induced graft polymerization that is applied to polymersubstrates in the form of a woven or non-woven fabric.

BACKGROUND ART

[0002] Radiation-induced graft polymerization is a technique by which asubstrate polymer is irradiated to form radicals and a polymerizablemonomer is grafted onto the radicals. Since functional radicals can beintroduced into various shapes of high-molecular weight compounds,radiation-induced graft polymerization is drawing increasing attentionthese days as a process for producing materials having a separatingcapability. The attention is especially significant when it comes tomethods of producing stock for air purifying chemical filters which haverecently seen increasing use in purifying the air in clean roomsemployed in precision electronics such as semiconductor fabrication andin the manufacture of pharmaceuticals, as well as to methods ofproducing stock for ion-exchange filters used in the production of purewater.

[0003] Polyolefinic high-molecular weight materials are considered to besuitable as polymeric substrates for radiation-induced graftpolymerization. Among these, polyethylene is considered the best stockfor radiation-induced graft polymerization. This is because polyethyleneis easier to crosslink but more resistant to decay after exposure toradiation as compared to other polyolefinic materials.Polyethylene-based substrates for radiation-induced graft polymerizationare well known in the form of film and hollow yarn, which are used asion-exchange membranes, cell diaphragms, air purifying materials,affinity separating membranes, water treating materials, deodorants,etc. A case of producing a cell diaphragm using a film treated byradiation-induced graft polymerization is disclosed in YUASA JIHO, 54,57-62 (1983) under the title “On Membranes Produced by Pre-irradiationGraft Polymerization”. The use of shaped polymers as water treatingmaterials is disclosed in Japanese Patent Public Disclosure Nos.111685/1993 and 111637/1993.

[0004] Forming woven or non-woven fabrics from the fibers of polymerssuch as polyolefins and polyesters and using them as filter stock is acommon practice; however, as far as the present inventors know,commercial use of polyethylene monofilament fiber as the raw materialfor filter stock in the form of a woven or non-woven fabric as a fiberaggregate has been very scarce. This is because the physical andchemical characteristics, such as the melting point and chemicalresistance, of polyethylene are inferior to those of other polyolefinicmaterials typified by polypropylene, so the use of polyethylenemonofilament fiber has not drawn much attention as a candidate forfilter stock. In fact, the polyethylene monofilament fiber hasoutstanding characteristics for radiation-induced graft polymerizationand it can be processed into woven or non-woven fabrics, into whichfunctional radicals are introduced by radiation-induced polymerization.However, the woven or non-woven fabric materials thus produced byradiation-induced graft polymerization do not have high enough physicalstrength, so they undergo permanent set strain, commonly called“failure”, and are unable to maintain sufficient strength anddimensional stability to function as filter stock. As the result,considerable difficulty has been encountered in molding them into apleated filter or the molded filter experiences increased pressure loss.

[0005] With a view to solving these problems, the present inventorsproposed the production of fiber having improved separating capabilityby applying radiation-induced graft polymerization to a core/sheathcomposite fiber (Japanese Patent Public Disclosure No. 199480/1996). Tomake the proposed fiber having improved separating capability, acomposite fiber using high-melting point polymers such as polyethylene(in the sheath) and polypropylene or polyethylene terephthalate (in thecore) is employed as the substrate for grafting and this enables athermal fusion method to be practiced at the stage of processing into anon-woven fabric. As a result, the physical strength of the core iscombined with the force of adhesion created at the points of contactbetween individual filaments and the fiber exhibits a very significantphysical strength.

[0006] A problem with this composite fiber is that graft polymerizationoccurs primarily in the sheath-forming polyethylene, so the sheathoccasionally separates from the core after graft polymerization tocreate gaps, in which processing chemicals stay to induce deteriorationof the fiber characteristics for the cleaning step in the manufacturingprocess. If an attempt is made to increase the graft ratio of thecomposite fiber taken as a whole, the graft ratio of the sheathincreases so much that its breakdown occurs, though on rare occasions.

DISCLOSURE OF THE INVENTION

[0007] The present inventors conducted intensive studies with a view toenhancing the strength of the stock that was to be formed by applying aradiation-induced graft polymerization treatment to a substrate in theform of a woven or non-woven fabric composed of polyethylenemonofilament fiber. As a result, it was found that when a substrate inthe form of a woven or non-woven fabric composed of polyethylenemonofilament fiber was combined with a reinforcement polymer having agreater strength and a slower rate of graft polymerization than thepolyethylene monofilament fiber, the physical strength of thepolyethylene woven or non-woven fabric material was enhanced; it wasalso found that by applying radiation-induced polymerization to saidmaterial, filter stock having improved capabilities and strengthcharacteristics was produced. It was further found that this filterstock had outstanding advantages that were quite unexpected as will beset forth hereinafter.

[0008] Thus, according to its first aspect, the present inventionrelates to a polymer substrate for radiation-induced graftpolymerization in the form of a woven or non-woven fabric that comprisesa woven or non-woven fabric composed of polyethylene fiber and areinforcement polymer having a greater strength and a slower rate ofgraft polymerization than said polyethylene fiber.

[0009] The invention also relates to filter stock that has functionalradicals introduced into the substrate for radiation-induced graftpolymerization according to the first aspect by radiation-induced graftpolymerization.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a perspective view showing the surface profile of thereinforced non-woven fabric material of the invention as subjected toradiation-induced graft polymerization;

[0011]FIG. 2 is a micrograph showing how a polymer substrate forradiation-induced graft polymerization according to the first aspect ofthe invention which comprises a non-woven fabric composed ofpolyethylene fiber and reinforcing polyethylene filaments knitted intothe non-woven fabric looks like on the fiber surface beforeradiation-induced graft polymerization;

[0012]FIG. 3 is a micrograph showing how the polymer substrate of FIG. 2looks like on the fiber surface after radiation-induced graftpolymerization;

[0013]FIG. 4 is a micrograph showing how a polymer substrate forradiation-induced graft polymerization according to the first aspect ofthe invention which comprises a non-woven fabric composed ofpolyethylene fiber and s reinforcing polyethylene net fused to bothsides of the non-woven fabric looks like on the fiber surface beforeradiation-induced graft polymerization;

[0014]FIG. 5 is a micrograph showing how the polymer substrate of FIG. 4looks like on the fiber surface after radiation-induced graftpolymerization;

[0015]FIG. 6 shows in conceptual form an apparatus for a continuousradiation-induced graft polymerization treatment that is used toimplement the method according to the sixth aspect of the presentinvention;

[0016]FIG. 7 shows in conceptual form another example of the apparatusfor a continuous radiation-induced graft polymerization treatment;

[0017]FIG. 8 is a schematic diagram of the equipment used to test thefilter stock produced by grafting functional grafts onto a non-wovenfabric according to the first aspect of the invention;

[0018]FIG. 9 shows by SEM and XMA a cross section of the high-densitypolyethylene fiber produced by radiation-induced graft polymerization inExample 4, FIG. 9a being a SEM micrograph of the fiber cross section andFIG. 9b an XMA micrograph showing the distribution of sulfur atoms inthe fiber cross section; and

[0019]FIG. 10 shows by SEM and XMA a cross section of the low-densitypolyethylene fiber produced by radiation-induced graft polymerization inComparative Example 3, FIG. 10a being a SEM micrograph of the fibercross section and FIG. 10b an XMA micrograph showing the distribution ofsulfur atoms in the fiber cross section.

BEST MODE FOR CARRYING OUT THE INVENTION

[0020] On the pages that follow, we describe the unexpected advantagesof the polymer substrate for radiation-induced graft polymerizationaccording to the first aspect of the invention. When woven or non-wovenfabrics which are a fiber aggregate are to be used as stock of filtersfor gases and liquids, filters are often produced by pleating the stock(i.e., folding the woven or non-woven fabric into pleats) in order toreduce pressure loss. In this case, a spacer is commonly insertedbetween sheets of the filter stock so that their distance is keptconstant at peaks, at valleys and in the flat region between a peak andan adjacent valley. The spacer is commonly made of metal or plastics. Inorder to keep a constant distance between the sheets of the filterstock, the spacer in most cases takes a wavy form and other regulararrangements of high and low spots.

[0021] In the case of chemical filters, it is very important that thewoven or non-woven fabric be exposed to uniform face velocity. Speakingof the conventional filter used to reject fine particles, even if theflow velocity is not uniform, more of the fluid flows throughhigh-velocity areas to get more of the fine particles trapped, whereuponan increased pressure loss occurs in those areas and the fluid flowstoward the areas of smaller pressure loss; as a result, the flowvelocity of the fluid becomes uniform by itself. On the other hand, thechemical filter has a gaseous component as the substance to be rejectedand even if it is rejected, there will be no corresponding increase inpressure loss. Therefore, with chemical filters, utmost care has beenexercised in spacer design in order to keep the face velocity constant.

[0022] This has unavoidably increased the number of high and low spotsthat serve to keep the distance between two woven or non-woven fabricsheets constant. However, if the high and low spots of the spacercontact the adjacent woven or non-woven fabric sheet, air is unable tohave access to the areas of contact and the effective filtration area ofthe filter decreases. In an extreme case, the non-effective filtrationarea has exceeded 20% of the total area of the woven or non-wovenfabric. Considering this point, it is often in the case of moldingpleated filters that the woven or non-woven fabric is folded to producemore pleats than are required theoretically but this has only led to ahigher production cost. The same problem exists with filters of aparallel-flow type.

[0023] If the woven or non-woven fabrics composed of polymer fiber aresubjected to radiation-induced graft polymerization, they undergodimensional changes in thickness, longitudinal and transversedirections. Since the substrate of the present invention is thecombination of a woven or non-woven fabric composed of polyethylenefiber and a reinforcement polymer having a greater strength and a slowerrate of graft polymerization than the polyethylene fiber, graftpolymerization proceeds at different speeds in the fiber andreinforcement areas, causing dimensional changes to occur at differentratios. As a result, the substrate acquires an undulated surface aftergraft polymerization.

[0024] Take, for example, the case where a substrate in the form of apolyethylene non-woven fabric is combined with a reinforcement polymerin the form of a polyethylene net that has a larger wire diameter (thatis, having a greater strength and a slower rate of graft polymerizationthan the polyethylene of which the substrate non-woven fabric iscomposed) and which is fused to the substrate. Upon radiation-inducedgraft polymerization, the substrate acquires undulations on the surfaceas shown in FIG. 1, in nwhich numeral 10 represents the fibrousnon-woven fabric material and 11 represents the netting.

[0025] As will be described later in the section of Examples, asubstrate non-woven fabric made of polyethylene fiber was intermeshedwith polyethylene filaments of 100 d (denier) in two crossed directions(Example 1) or had a polyethylene net of a larger wire diameter than thepolyethylene fiber fused to both sides of the non-woven fabric (Example2). The two substrates were subjected to radiation-induced graftpolymerization and their surface profiles are shown by micrograph inFIG. 2 (before grafting) and FIG. 3 (after grafting) for the case ofExample 1 and in FIG. 4 (before grafting) and FIG. 5 (after grafting)for the case of Example 2. Obviously, the substrates according to theinvention had undulations formed on the surface by application of graftpolymerization.

[0026] Thus, the stock produced by performing graft polymerization onthe substrate non-woven fabric of the invention possesses undulations onthe surface and by pleating it, pleats are formed in each sheet offilter stock such that their distance is kept constant without using aspacer. Therefore, if an ion-exchange chemical filter is fabricated bypleating the filter stock produced by graft polymerization in accordancewith the invention, the distance between two filters is kept constant atpeaks, at valleys and in the flat region between a peak and an adjacentvalley to create a uniform flow velocity, enabling efficient consumptionof the overall ion-exchange capacity of the filter assembly. Inaddition, since the step of inserting spacers is eliminated, the filtermanufacturing process is simplified and made cost-effective. As afurther advantage, graft polymerization has also occurred in thereinforcement area, though at small graft ratio. Therefore, both thesubstrate non-woven fabric and the reinforcement possess ion-exchangegroups and the filter stock taken as a whole has an even greater gasadsorption capacity and, hence, a longer operating life. If some specialmeasure is taken to secure the joining between the reinforcement and thepolyethylene fiber, it also becomes possible to suppress the dislodgingof fiber fragments from the woven or non-woven fabric.

[0027] The polyethylene monofilament fiber which is a component of thewoven or non-woven fabric according to the invention has preferably athickness that typically ranges from several to several tens of deniers.

[0028] In the present invention, a polymer having a higher strength anda slower rate of radiation-induced graft polymerization than thepolyethylene monofilament fiber which is a component of the woven ornon-woven fabric, which is used as a reinforcement of said fabric andvarious types of reinforcements can be used for this purpose, includingpolyethylene yarns and yarn-like products thicker than the polyethylenefiber, as well as articles produced by processing them, as exemplifiedby sheeting such as nets and polyethylene films, and torn and perforatedversions of sheeting. These reinforcements have larger wire diametersand smaller surface areas per unit weight than the polyethylenemonofilament fiber which is a component of the woven or non-woven fabricand, hence, they are subject to slower radiation-induced graftpolymerization.

[0029] Polymeric materials are generally known to deteriorate inphysical strength when subjected to radiation-induced graftpolymerization. However, speaking of the above-mentioned reinforcementpolyethylene of the invention, it has a larger wire diameter than thepolyethylene monofilament fiber of which the woven or non-woven fabricis made, so even at the end of the radiation-induced graftpolymerization reaction, radiation-induced graft polymerization has notproceeded beyond the neighborhood of the surface of the reinforcementpolyethylene and its physical strength will not be greatly impaired.Therefore, due to its combination with the reinforcement describedabove, the polyethylene material of the invention maintains its physicalstrength without experiencing substantial decrease afterradiation-induced graft polymerization.

[0030] Various forms of reinforcement polyethylene can be compositedwith the polyethylene fiber and processed into reinforced woven ornon-woven materials by several methods, and if the reinforcement ispolyethylene yarns or yarn-like products, it is convenient to knit theminto the woven or non-woven fabric made of the polyethylene monofilamentfiber. If the reinforcement is a polyethylene film or net, it isconveniently bonded to the surface of the polyethylene woven ornon-woven fabric. Thermal fusion is a preferred bonding method. It isalso possible to form a non-woven fabric material by interlacing the netor film with the polyethylene fiber.

[0031] While various methods well known in the art may be used toproduce the woven or non-woven fabric made of the polyethylenemonofilament fiber, it is preferred to adopt methods that take specialmeasures for keeping the fiber aggregate intact as by needle punching orembossing such as spot welding.

[0032] The foregoing description mainly concerns forming the woven ornon-woven fabric of polyethylene fiber and combining it with thereinforcement polyethylene but it should be understood that the conceptof the invention is also applicable to other polymeric materials. To bespecific, polymer fibers other than the polyethylene fiber can be usedto make the woven or non-woven fabric; in addition to or alternatively,the woven or non-woven fabric may be combined with reinforcementpolymeric materials having greater strengths and slower rates of graftpolymerization than the fibers of which the woven or non-woven fabric ismade. When the substrate for graft polymerization that is made of thiscombination is subjected to radiation-induced graft polymerization, thewoven or non-woven fabric portion and the reinforcement portion undergografting at different ratios and, hence, experience dimensional changesby different amounts as already explained in connection with thepolyethylene material; thus, after the graft polymerization, thesubstrate similarly acquires undulations on the surface and can bepleated without using a separator. As for the reinforcement, itundergoes only a low degree of graft polymerization, so its strengthwill not be greatly impaired and this contributes to retaining thephysical strength of the substrate.

[0033] Thus, according to its second aspect, the present inventionrelates to a polymer substrate for radiation-induced graftpolymerization in the form of a woven or non-woven fabric that comprisesa woven or non-woven fabric composed of polymer fiber and areinforcement polymer having a greater strength and a slower rate ofradiation-induced graft polymerization than said polymer fiber.

[0034] Speaking of the materials that can be used in the second aspectof the invention, the polymer fiber which composes the woven ornon-woven fabric may be selected from among halogenated polyolefins suchas polyvinyl chloride and polyvinyl fluoride, and the polymeric materialwhich composes the reinforcement may be selected from among halogenatedpolyolefins, polyethylene terephthalate, polyurethane fiber, vinylonfiber, cellulose fiber and inorganic fibers such as glass fiber.

[0035] In the second aspect of the invention, various parameters ofundulations that are formed upon application of graft polymerization tothe substrate for graft polymerization, such as their height (thedistance between pleats formed by pleating) and number, can becontrolled by adjusting factors such as the graft ratios of thereinforcement and the substrate polymer fiber, the shape of thereinforcement, its areal density and thickness.

[0036] When forming the substrate woven or non-woven fabric from thepolymer fiber, another polymer fiber having a slower rate ofradiation-induced graft polymerization than said polymer fiber may alsobe used and combined with said polymer fiber to form the substrate wovenor non-woven fabric and this is another effective way to enhance thestrength of the substrate woven or non-woven fabric afterradiation-induced graft polymerization. For example, polyethylene fiberand polyethylene terephthalate fiber (which is known to be littlereactive in radiation grafting) may be mixed at a ratio of about 80:20to form a non-woven fabric, thereby producing the substrate forradiation-induced graft polymerization. In this substrate, thepolyethylene fiber and the polyethylene terephthalate fiber are randomlyinterlaced and when it is subjected to radiation-induced graftpolymerization reaction, only the polyethylene part of the interlacedfibers undergoes the graft reaction and the polyethylene terephthalatefiber remains almost unreacted. Hence, the strength of the substratenon-woven fabric is retained by the polyethylene terephthalate fiber inwhich no graft reaction has occurred. In this case, however, thepolyethylene fiber and the polyethylene terephthalate fiber are arrangedrandomly (i.e., uniformly), so the substrate acquires only fewundulations after the radiation-induced graft polymerization reactionand if it is to be used as a pleated filter, a spacer has to be insertedbetween two filters as is conventionally the case.

[0037] Thus, according to its third aspect, the present inventionrelates to a polymer substrate for radiation-induced graftpolymerization in the form of a woven or non-woven fabric which isformed by mixing polymer fiber with another polymer fiber having aslower rate of radiation-induced graft polymerization than said polymerfiber.

[0038] Speaking of the materials that can be used in the third aspect ofthe invention, the polymer fiber may be selected from among polyethyleneand halogenated polyolefins such as polyvinyl chloride and polyvinylfluoride, and the “another polymer fiber” having a slower rate ofradiation-induced graft polymerization may be selected from amonghalogenated polyolefins, polyethylene terephthalate, polyurethane fiber,vinylon fiber, cellulose fiber and inorganic fibers such as glass fiber.Two or more of these fibers may be used as the “another polymer fiber”.

[0039] The present inventors continued their studies in order to findanother technique for enhancing the strength of the substrate forradiation grafting which was formed of a woven or non-woven fabric ofpolyethylene fiber. As a result, they found that by performing aradiation-induced graft polymerization treatment on the woven ornon-woven fabric of polyethylene fiber in such a way that the graftreaction would occur only in the surface of the fiber whereas its centerwould remain unaffected by the graft reaction, filter stock of highphysical strength in the form of a radiation graft treated woven ornon-woven fabric could be provided using the polyethylene monofilamentfiber heretofore considered difficult to utilize.

[0040] Thus, according to its fourth aspect, the present inventionrelates to a radiation graft treated material in the form of a woven ornon-woven fabric material that is composed of polyethylene monofilamentfiber of which only the surface has undergone radiation-induced graftpolymerization but of which the center remains unaffected by grafting.

[0041] Radiation-induced graft polymerization is generally performed inorder to introduce functional radicals such as ion-exchange groups intosubstrates, so one of the general design considerations in the art isthat from the viewpoint of increasing the density of radicals in thesubstrate by introducing as many functional radicals as possible, graftpolymerization desirably proceeds deeper into the bulk of the substrate.On the other hand, it is known that the physical strength of polymericmaterials decreases if it is subjected to a radiation-induced graftpolymerization treatment. Particularly in the case of introducinghydrophilic groups such as ion-exchange groups into the substrate bymeans of radiation-induced graft polymerization, water is attracted bythe ion-exchange groups and the physical strength of the substratebecomes considerably lower than before grafting was performed.

[0042] In the radiation graft treated stock according to the fourthaspect of the invention, graft polymerization has occurred only in thesurface of a fiber cross section and no grafting has occurred in thecenter, so the fiber center which remains unaffected by graftingmaintains the pre-grafting value of physical strength. Hence, one canproduce fiber stock into which functional radicals have been introducedby means of radiation-induced graft polymerization without greatlydeteriorating the physical strength of the fiber taken as a whole. Inthe radiation graft treated material according to the fourth aspect ofthe invention, the center of the fiber which remains unaffected bygrafting performs the function of maintaining physical strength whereasthe surface of the fiber which has undergone graft polymerizationperforms an added function such as an ion-exchange capability that hasbeen imparted by graft polymerization.

[0043] Thus, according to the fourth aspect of the invention, one canmaintain high levels of physical strength that have been consideredunattainable by the conventional radiation graft treatment and radiationgraft treated materials having outstanding physical strength and highgraft ratio can be produced using substrate woven or non-woven fabricsmade of polyethylene monofilament fiber which have heretofore beenunsuitable for use as the substrate for radiation-induced graftpolymerization on account of difficulties such as low fiber strength andthe occurrence of failure.

[0044] According to the fourth aspect of the invention, only the surfaceof the fiber “has undergone radiation-induced graft polymerization”, andthis means that not all of the fiber cross section has undergonegrafting but an area that has been unaffected by grafting remains in thecenter of the fiber cross section. If the proportion of the areaunaffected by grafting is high, the graft treated fiber stock has highstrength but the overall graft ratio is low. Conversely, if theproportion of the area unaffected by grafting is low, high graft ratiois achieved but the graft treated fiber stock has low strength. Thedepth of grafting depends on the intended use of the stock and isdetermined by the balance between the desired values of graft ratio andstock's strength. Typically, fiber stock that has been affected bygrafting in an area extending from the fiber surface down to a point inits interior between ⅓ and ⅔ of its thickness is preferred from theviewpoint of attaining a balance that can maintain high strength whileachieving high graft ratio.

[0045] Allowing the radiation-induced graft polymerization reaction toproceed such that grafting occurs only in the surface of the substratefiber but that its center remains unaffected by grafting and determiningthe depth of grafting are tasks the skilled artisan can accomplishempirically by appropriately choosing various parameters such as, forexample, the kind of the substrate fiber, its size, the method ofirradiation, its intensity, its duration, the method of graftpolymerization reaction and the conditions therefor.

[0046] Polyethylenes are classified by production process into twotypes, the low-density polyethylene and the high-density polyethylene.Low-density polyethylenes have smaller degrees of crystallizationbetween 50 and 60% and contain such a large portion of the amorphousstate that during the graft reaction, monomers diffuse at a high enoughrate to increase the likelihood of graft polymerization to proceed intothe fiber's interior. In contrast, high-density polyethylenes havingspecific gravities greater than 0.94 present larger degrees ofcrystallization between 80 and 90% and contain such a small portion ofthe amorphous state that they are favorable to the purpose of ensuringthat the graft reaction will occur only in the fiber surface and nearbyareas and will not proceed deeper into the interior. Further,high-density polyethylenes permit the generation of many radicals in thecrystal that can be utilized in the graft reaction so that they offeradvantages such as increased strength after graft polymerization andhigher ultimate graft ratio. Hence, the high-density polyethylene ishighly suitable for use as the substrate in producing the radiationgraft treated stock of the invention.

[0047] Again, the technical concept for the fourth aspect of theinvention is applicable not only to the stock based on the woven ornon-woven fabric made of polyethylene monofilament fiber but also toother polymeric materials. Thus, according to its fifth aspect, thepresent invention relates to a radiation graft treated material in theform of a woven or non-woven fabric material that is composed of polymermonofilament fiber of which only the surface has undergoneradiation-induced graft polymerization but of which the center remainsunaffected by grafting. Examples of the substrate fiber that can be usedin the fifth aspect of the invention to compose the woven or non-wovenfabric material include not only polyethylene but also halogenatedpolyolefins such as polyvinyl chloride and polyvinyl fluoride.

[0048] The substrate for graft polymerization according to the presentinvention can be subjected to radiation-induced graft polymerization bytwo methods, the pre-irradiation process in which the substrate is firstirradiated and then brought into contact with a polymerizable monomerand the simultaneous irradiation process in which the substrate isirradiated in the presence of a polymerizable monomer. Either method canbe adopted in the present invention.

[0049] The methods of graft polymerization may be classified as followsby the manner in which the polymerizable monomer is brought into contactwith the substrate. The method in which the irradiated substrate is keptimmersed in the monomer solution as it is subjected to graftpolymerization is called “liquid-phase graft polymerization” and in thiscase, the reaction temperature and time are preferably within the rangesof 20-60° C. and 2-10 hours, respectively. The liquid-phase graftpolymerization process is capable of uniform graft polymerization buthas the problem of consuming large amounts of processing chemicals.

[0050] The method in which the irradiated substrate is brought intocontact with the monomer vapor is called “vapor-phase graftpolymerization”. While this method is only applicable to monomers havingcomparatively high vapor pressures and has high likelihood for causinguneven grafting, it has the advantage of allowing the substrate to beobtained in a dry state after graft polymerization. In this case, thereaction temperature and time are preferably within the ranges of 20-80°C. and 2-10 hours, respectively.

[0051] Either of the polymerization methods described above can beapplied to implement the present invention.

[0052] The method in which a specified amount of monomer is imparted tothe irradiated substrate and reaction is performed in vacuum or an inertgas to effect graft polymerization is called “impregnation graftpolymerization”; in this case, the reaction temperature and time arepreferably within the ranges of 20-60° C. and 0.2-8 hours, respectively.The impregnation graft polymerization process is economical since almostall monomers used undergo the reaction and very little of the processingchemicals remain unreacted. In addition, the substrate is obtained in adry state after graft polymerization, giving advantages such as the easeof handling the substrate and smaller emission of liquid wastes. Thismethod is particularly effective in the case where porous materials suchas woven and non-woven fabrics are used as the substrate for grafting.

[0053] For the reason to be mentioned below, the impregnation graftpolymerization process is very effective if it is especially used in thefourth and fifth aspects of the invention. In impregnation graftpolymerization, the irradiated substrate having radicals is contacted bya monomer to initiate the polymerization reaction and as it progresses,the radicals move from the irradiated substrate onto the monomer whichthen starts to polymerize by itself. Because of this phenomenon, as thestage of reaction proceeds into the second half, the monomer grows inmolecular weight and presents difficulty in impregnating or otherwisegetting into the substrate fiber, thereby increasing the chance ofensuring that the graft reaction will not go deeper than the surface ofthe substrate fiber. On the other hand, the liquid-phase graftpolymerization process uses by far larger amounts of monomers than thesubstrate and they remain unreacted in high proportions even in thesecond half of the reaction and will diffuse or permeate into the bulkof the fiber; therefore, in order to obtain graft treated stock in whichonly the surface of the fiber has undergone radiation-induced graftpolymerization according to the fourth and fifth aspects of theinvention, process conditions such as the time and temperature for thegraft reaction need be controlled positively.

[0054] Generally speaking, it is preferred that the polyethylenemonofilament fiber used in the invention as a component of the substratein the form of a woven or non-woven fabric has a fineness from severalto several tens of deniers.

[0055] While various methods well known in the art may be employed toprepare the woven and non-woven fabrics for use as the substrate forgrafting in the present invention, it is preferred to adopt methods thattake various measures to prevent disintegration of the fiber aggregateas by needle punching or embossing such as spot welding.

[0056] The polymerizable monomer to be introduced into the substrate byradiation-induced graft polymerization is one having various functionalradicals in itself or another that is first grafted to the substrate andthen subjected to a secondary reaction to thereby introduce functionalradicals.

[0057] Consider, for example, the case of producing ion-exchange filterstock by the present invention. Using monomers having ion-exchangegroups such as acrylic acid, methacrylic acid, sodiummethylenesulfonate, sodium metallylsulfonate, sodium allylsulfonate andvinylbenzyl trimethyl ammonium chloride, radiation-induced graftpolymerization may be performed so that functional radicals are directlyintroduced into the substrate fiber, thereby producing ion-exchangefilter stock.

[0058] Examples of the monomer that is first subjected toradiation-induced graft polymerization and then to a secondary reactionto introduce ion-exchange groups include acrylonitrile, acrolein,vinylpyridine, styrene, chloromethylstyrene and glycidyl methacrylate.In an exemplary case, glycidyl methacrylate is introduced into thesubstrate non-woven fabric by radiation grafting and then reacted with asulfonating agent such as sodium sulfite to introduce sulfone groups,thereby producing ion-exchange fiber. If desired, an amino-introducingcompound such as diethanolamine may be used in combination with glycidylmethacrylate to introduce ion-exchange groups such as quaternaryammonium and a tertiary amino group.

[0059] Primary applications of the present invention include a methodfor producing ion-exchange filter stock; the invention finds use inother applications such as heavy metal adsorbents having a chelategroup, catalysts, affinity chromatographic carriers, etc.

[0060] It should be noted that one of the above-described first to thirdaspects of the invention can be combined with the fourth or fifthaspect. To be more specific, a polymer substrate for radiation-inducedgraft polymerization in the form of a woven or non-woven fabric thatcomprises a woven or non-woven fabric composed of polymer fiber and areinforcement polymer having a greater strength and a slower rate ofradiation-induced graft polymerization than said polymer fiber or apolymer substrate for radiation-induced graft polymerization in the formof a woven or non-woven fabric which is formed by mixing polymer fiberwith another polymer fiber having a slower rate of radiation-inducedgraft polymerization than said polymer fiber is subjected toradiation-induced graft polymerization reaction such that only thesurface of the polymer fiber undergoes radiation grafting to produce aradiation graft treated material of improved strength.

[0061] The present inventors also found a very effective method forperforming radiation-induced graft polymerization on woven or non-wovenfabric materials. Details are given below.

[0062] As already mentioned, methods of radiation-induced graftpolymerization are classified into simultaneous irradiation graftpolymerization and pre-irradiation graft polymerization. While bothmethods can be commercialized, the pre-irradiation method yields lesshomopolymer as the result of polymerization of the monomer to be graftedand is suitable for use in water treatments to produce materials for theproduction of pure water or in gas treatments to produce materials forair purification or production of clean air.

[0063] The pre-irradiation graft polymerization methods are classifiedinto liquid-phase graft polymerization and gas-phase graftpolymerization depending on whether the monomer to be brought intocontact with the irradiated substrate is liquid or gas. Gas-phase graftpolymerization uses less monomer and is economical; on the other hand,it is only applicable to monomers of high vapor pressure and unevengrafting tends to occur in the product. These problems are absent fromliquid-phase graft polymerization, which is applicable to a broad rangeof monomers and, hence, is a general-purpose method.

[0064] However, if a highly porous substrate such as a woven ornon-woven fabric is to be subjected to pre-irradiation liquid-phasegraft polymerization, uniform grafting in high graft ratio is difficultto achieve and the product of graft polymerization does not have thedesired physical stability unless the oxygen trapped in the pores in thesubstrate is adequately removed during irradiation or graftpolymerization reaction. What is more, a large amount of cleaningchemical must be used after graft polymerization to wash off theunwanted monomer trapped in the pores in the substrate and the treatmentof the resulting liquid waste is very costly.

[0065] As a method of solving these problems, the present inventorsproposed a new approach of graft polymerization in U.S. Pat. No.1,933,644. It is a method of radiation-induced graft polymerizationcharacterized by separating irradiation and graft polymerizationreaction such that a pre-irradiated substrate is impregnated with aspecified amount of monomer solution and placed under vacuum to removethe residual oxygen before graft polymerization reaction is performed ina gas phase under vacuum. In this method, the monomer solution adheringto the substrate serves as a source of monomer vapor in the gas-phasegraft reaction, the product of grafting can be finished in a dry stateand the graft ratio can be easily controlled. For these and otherfeatures, the method was a unique technology possessing the advantagesof both the liquid-phase and gas-phase graft polymerization methods.What is more, since the step of irradiation is separated from the stepof graft polymerization reaction, the performer of graft polymerizationreaction need not possess a source of irradiation for himself and thisrenders the patented method suitable for small-lot production.

[0066] This method, however, has had the following problems. Thedistribution of radicals generated in the substrate by irradiationdepends on the energy of radiation being applied but it is generallyuniform in the substrate. If irradiation is performed in a separate stepfrom raft polymerization reaction, the radicals in the non-crystallinearea disappear and only the radicals generated in the crystals areutilized in the graft polymerization reaction. Therefore, the efficiencyof radical utilization is small and high radiation exposure has beennecessary to achieve satisfactory graft ratio.

[0067] In addition, if the interval from irradiation to graftpolymerization reaction increases, oxygen may leak from the container ofthe irradiated substrate depending on the constituent material of whichit is made and this increases the chance of radicals to decrease innumber or deteriorate in quality. As a further problem, the stage oftransition from the storage of the irradiated substrate to the graftpolymerization reaction requires a lot of manpower in operations such asthe recovery of the irradiated substrate from its container andreplacement and installation of the recovered substrate onto thereactor; in addition, the substrate becomes exposed to air during thetransition, again increasing the chance of decrease and deterioration ofradicals.

[0068] In graft polymerization, the progress of the reaction is moreadversely affected by the presence of oxygen than in the step ofirradiation, so oxygen must be removed from the substrate once exposedto air. In order to achieve adequate removal of the oxygen trapped intothe pores in the substrate, the step of evacuation has been necessary.Hence, the graft polymerization reactor must be a closed vesselcomparable to vacuum chambers and this has been a major obstacle to theeffort to implement a continuous treatment process.

[0069] Under these circumstances, a strong need has been felt to developa method by which a long web of woven or non-woven fabric material canbe subjected to a radiation-induced graft polymerization treatmentcontinuously in a large volume.

[0070] As a result of the intensive studies made to develop a methodcapable of solving the above-mentioned problems, the present inventorsfound that substrates, in particular, long webs of woven or non-wovenfabric materials could be subjected to efficient radiation-induced graftpolymerization by ensuring that exposure of the substrate to electronbeams, impregnation of the irradiated substrate with a monomer and graftpolymerization reaction would be performed continuously in a nitrogenatmosphere. Thus, according to its sixth aspect, the present inventionrelates to a method for performing radiation-induced graftpolymerization on substrates in the form of long webs of woven ornon-woven fabric, comprising the steps of exposing a substrate in theform of a woven or non-woven fabric as a fiber aggregate to electronbeams in a nitrogen atmosphere, contacting the irradiated substrate witha specified amount of monomer in a nitrogen atmosphere, and subjectingthe monomer and the substrate in mutual contact to graft polymerizationin a nitrogen atmosphere, said first through third steps being performedin succession. If desired, said first step is preferably preceded by apreliminary step of replacing the air in the substrate with nitrogengas.

[0071] Electron beams are a preferred example of the radiation source tobe employed in the sixth aspect of the invention. If γ-rays are to beused, a roll of substrate in the form of a woven or non-woven fabric isplaced on an irradiation platform and irradiated with γ-rays undercooling. However, this operation requires a lot of manpower and thesubstrate that can be treated per batch can be no longer than severalhundred meters. In exposure to electron beams, the radiation has a smallpenetrating power but its dose rate is sufficiently large that the wovenor non-woven fabric can be unrolled, irradiated and rewound at fastspeed. Since no problems occur if the length of the substrate is furtherincreased, the exposure to electron beams is suitable for high-volumeproduction.

[0072] In the sixth aspect of the invention, the preferred conditionsfor exposure to electron beams are a voltage in the range of 100 keV-500keV, an electron beam current in the range of 3 mA-50 mA, and anexposure in the range of 30 kGy-200 kGy. If the indicated parameters arewithin these ranges, radicals are uniformly generated on both sides ofordinary woven or non-woven fabrics and the substrate can be transportedat a high enough speed to enable high-volume production.

[0073] In the method according to the sixth aspect of the invention, thestep of irradiation is continuous to the step of graft polymerizationreaction, so the radicals generated by irradiation can be utilized inthe graft reaction before they disappear. Therefore, the requiredexposure, or dose of irradiation, can be reduced to ¼-{fraction (1/7)}of the conventionally required value.

[0074] In the sixth aspect of the invention, a nitrogen atmosphere forirradiation can be established by various methods well known in the artand introducing liquid nitrogen into the system is a preferred methodsince it fulfills two functions simultaneously, removal of oxygen andprevention of temperature elevation during irradiation. The temperatureof the substrate being exposed to electron beams depends on substrateshape and the interval between the exposure and the graft polymerizationreaction and it is generally preferred to lie between −50° C. and +50°C. According to the method of the invention, the irradiated substrate isnot exposed to air at any point of the time prior to graftpolymerization, so the amount of oxygen that may enter the substrate canbe reduced to the least possible level. In addition, there is no need touse a closed vessel ass the graft polymerization reactor and evacuateit; this enables the overall treatment process to be performedcontinuously.

[0075] In the method according to the sixth aspect of the invention, theoxygen present in the substrate in the form of a woven or non-wovenfabric must be removed before irradiation and to this end, nitrogenreplacement is preferably performed as a preliminary step. The oxygen inthe substrate can also be removed by blowing nitrogen against thesubstrate during the step of irradiation and, if this is done, thepreliminary step of nitrogen replacement can be obviated. However, inorder to apply the method of the invention to various shapes of thesubstrate in the form of a woven or non-woven fabric, it is preferred toinclude the preliminary step of nitrogen replacement.

[0076] Nitrogen replacement can be accomplished by a process consistingof evacuating the system and introducing nitrogen into it. The number oftimes this process is performed can be determined by various factorsincluding the shape and length of the substrate to be treated. In thepreliminary step of nitrogen replacement, evacuation is preferablyperformed within a closed vessel but due to the absence of the need tocontrol the oxygen concentration as strictly as in the graftpolymerization reaction, providing a chamber capable of maintaining aconstant vacuum state in the continuous apparatus will suffice. Byperforming the process of evacuation and nitrogen introduction inseveral cycles, the concentration of oxygen in the pores in thesubstrate can easily be reduced to the desired level and below. Thedegree of vacuum that is necessary to achieve adequate nitrogenreplacement in the method of the invention and the amount of nitrogenthat need be introduced for this purpose vary greatly with the shape ofthe substrate, its constituent material, the speed at which it istransported and other factors; generally speaking, nitrogen replacementcan be accomplished by evacuating the system to 20-30 mmHg and thenintroducing nitrogen into the system until its pressure becomes equal toone atmosphere. This process may be performed twice or three times inorder to further lower the oxygen concentration in the system.

[0077] In the method according to the sixth aspect of the invention, thesecond step of bringing the substrate into contact with a specifiedamount of monomer is provided between the step of irradiation and thestep of graft polymerization reaction. This second step is for ensuringthat a monomer in the amount used by the graft polymerization reactionis preliminarily imparted to the irradiated substrate. In this secondstep, the monomer in excess of the amount used by the graftpolymerization reaction is removed from the substrate, thus eliminatingthe need to remove the excess monomer from the substrate after the graftpolymerization reaction ends. In the conventional method in which theirradiated substrate is subjected to liquid-phase graft polymerizationas it is immersed in a monomer solution, the graft ratio has beencontrolled by reaction temperature and time. According to the method ofthe invention, the amount of the monomer to be imparted to the substrateis controlled, so compared to the method of control that solely dependson reaction temperature and time, the graft ratio can be controlled in amore consistent manner.

[0078] The substrate can be brought into contact with a specified amountof monomer by various methods known in the art. If a longer product lifeis preferred as in the case where the product is used as a chemicalfilter, high graft ratio is preferred and this end can conveniently bemet by immersing the substrate in the monomer solution. However, if thesubstrate is immersed in the monomer solution and later recovered, themonomer solution deposited is at least five times as heavy as thesubstrate and “sag” will occur. In order to bring a specified amount ofmonomer into contact with the substrate in the method of the invention,an excess monomer solution has to be squeezed off, for example, withrollers. Of course, any other methods well known in the art may be usedto remove the excess monomer solution.

[0079] The substrate to which the specified amount of monomer has beenimparted is then subjected to the step of graft polymerization. In themethod of the invention, graft polymerization is performed by heatingthe monomer-impregnated substrate to a graft polymerization temperaturebetween 20 and 80° C. in a nitrogen atmosphere. In order to attain highgraft ratio within a short time, the oxygen concentration in thereaction atmosphere is desirably maintained at 1000 ppm or belowalthough its exact value depends on the graft polymerization reactiontemperature. The oxygen concentration, the dose of radiation and thegraft polymerization reaction temperature may be adjusted as appropriateto attain a specified reaction rate and ultimate graft ratio. In thepresent invention, the substrate is generally heated at a graftpolymerization temperature within the stated range for 5 minutes to 5hours, preferably for 5 minutes to 1 hour, most preferably for 10minutes to 30 minutes, thereby allowing the graft polymerizationreaction to proceed.

[0080]FIG. 6 shows in conceptual form an apparatus for implementing themethod according to the sixth aspect of the invention. The apparatusconsists of a series arrangement of the following units: a nitrogenreplacement vessel 1 for replacing the air in the substrate woven ornon-woven fabric by nitrogen gas; an electron beam exposing unit 3 forapplying electron beams to the substrate in a nitrogen atmosphere; amonomer impregnating vessel 4 for bringing the irradiated substrate intocontact with a monomer in a nitrogen atmosphere; and a graftpolymerization vessel 5 for grafting the monomer onto the substrate in anitrogen atmosphere. A roll of the substrate 2 in the form of a woven ornon-woven fabric is unwound and passed through those units in successionand after the graft polymerization treatment ends, the substrate isrewound into a roll. To perform these operations, a sheeting transfermeans is provided in the apparatus and it consists of a sheet unwinder7, sheet transport rolls 8 and a sheet rewinder 9. As already mentioned,the step of nitrogen replacement can be omitted and if this is the case,the nitrogen replacement vessel 1 is dispensed with.

[0081] In the apparatus for continuous treatment of the substrate inlong web form which is used in the invention, the long web of substrateis transported through the apparatus at a suitable speed so that it issubjected to a continuous treatment in successive steps. The suitabletransport speed is generally in the range of 1-20 m/min, preferably 2-15m/min, most preferably 2-10 m/min.

[0082] Examples of the substrate that can be subjected toradiation-induced graft polymerization treatment by the method accordingto the sixth aspect of the invention include long web materials such aswoven fabrics, non-woven fabrics and foams that are typically made ofpolyolefins such as polyethylene and polypropylene or halogenatedpolyolefins such as polyvinyl chloride.

[0083] In the method according to the sixth aspect of the invention, along web sheet of woven or non-woven fabric that is to be subjected toradiation-induced graft polymerization treatment (i.e., a long web sheetfor radiation-induced graft polymerization) may be installed as such ina reaction apparatus of the type shown in FIG. 6 but in this case, boththe leading and trailing end portions of the long web sheet remainunreacted after subsequent treatments. First, consider the case where along web sheet is mounted in the reaction apparatus shown in FIG. 6, therespective steps of the method are adjusted to have the necessaryreaction conditions and reaction is carried out as the long web sheet isrewound by the take-up roll 9. In this case, not all of the long websheet that was initially mounted in the reaction apparatus is subjectedto the entire process of the radiation-induced graft polymerizationreaction but that part of the long web sheet which lied beyond theirradiation zone 3 at the start of operation remains unreacted althoughit is eventually taken up by the roll 9. Next, consider the case oftreating two or more rolls of long web sheet continuously, with asubsequent roll being mounted after the treatment of a preceding roll.If reaction is resumed after the trailing end of the preceding long webis joined to the leading end of the subsequent long web sheet, no partof the joint will remain unreacted. However, in the case of stopping thereaction apparatus after all rolls have been treated, the whole sheet ofthe last roll is unwound from the sheet unwinder 7, the reactionapparatus is switched off and then all sheeting is rewound by thetake-up roll 9. Hence, that part of the long web sheet which lied beyondthe polymerization reaction zone 5 at system shutdown is again taken upby the roll 9 without having been subjected to the entire process of theradiation-induced graft polymerization reaction. In short, the trailingand leading end portions of long web sheets for radiation-induced graftpolymerization remain unreacted and must be cut off before the treatedlong webs are used as filter stock but then the product yield lowers.Consider, for example, the case where 500-m long rolls of long web sheetfor radiation-induced graft polymerization are passed through a reactionapparatus 50 m long. That part of the sheet of the first roll which is50 m long as measured from its leading end and that part of the sheet ofthe last roll which is 50 m long from its trailing end are both rewoundwithout passing through the entire reaction process and should berejected as defectives.

[0084] One method of solving these problems would be by using atransport long web material (dummy sheet) that is as long as or slightlylonger than the reaction apparatus. Such a dummy sheet is mounted overthe transport line through the reaction apparatus, with its leading endbeing mounted on the take-up roll 9 and its trailing end positioned inthe nitrogen replacement vessel 1. Then, a roll of long web sheet forradiation-induced graft polymerization is set on the sheet unwinder 7and its leading end is joined to the trailing end of the dummy sheetwithin the nitrogen replacement vessel 1. Thereafter, the respectivesteps in the reaction apparatus are adjusted to have the necessaryreaction conditions and the dummy sheet is rewound by the take-up roll9, whereupon the long web sheet for radiation-induced graftpolymerization progresses through the reaction apparatus. This approachensures that all part of the long web sheet for radiation-induced graftpolymerization including its leading end can be subjected to the wholeprocess of the radiation-induced graft polymerization reaction. Ifdesired, another dummy sheet may be joined to the trailing end of thelong web sheet for radiation-induced graft polymerization; in this case,all part of the long web sheet for radiation-induced graftpolymerization can be subjected to the whole process of theradiation-induced graft polymerization reaction and taken up by the roll9; then, the reaction apparatus is turned off and the long web sheetthat has been taken up after the radiation-induced graft polymerizationtreatment is cut off from the dummy sheet and taken out of the reactionapparatus. In the next place, the leading end of the dummy sheet left inthe reaction apparatus is mounted on the take-up roll 9 and a smallamount of it is rewound by the take-up roll 9 until its trailing endportion is positioned within the nitrogen replacement vessel 1;subsequently, another roll of long web sheet for radiation-induced graftpolymerization is set on the sheet unwinder 7 and the above-describedprocedure is repeated by joining its leading end to the trailing end ofthe dummy sheet, adjusting the respective steps in the reactionapparatus to have the necessary reaction conditions and turning thetake-up roll 9, whereupon all part of the long web sheet can besubjected to the whole process of the radiation-induced graftpolymerization treatment. As another advantage, this approach eliminatesthe need to mount a long web sheet over the entire transport line in thereaction apparatus each time roll replacement is done and this leads tosubstantial reduction of manpower.

[0085] Examples of the transport long web sheet (dummy sheet) that canbe used for the purposes described above include nonporous films, nets,as well as woven and non-woven fabrics that are made from halogenatedpolyolefins, polyethylene terephthalate, polyurethane fiber, vinylonfiber, cellulose fiber and inorganic fibers such as glass fiber.

[0086] In the method according to the sixth aspect of the invention, theradiation-induced graft polymerization treatment can be performed usinglong webs of woven or non-woven fabrics made of ordinary polymermonofilament fiber. A problem with this case is that the substrate fiberis embrittled by graft reaction and the long web of woven or non-wovenfabrics may potentially rupture while it is being transported throughthe reaction apparatus by the rewinding action of the take-up roll. Thisproblem can be solved by joining the substrate in the form of a long websheet of woven or non-woven fabric to a sheet transport support material(transport sheet) equal in length to the substrate and then rewindingthe two sheets together by means of the take-up roll so as to transportthe substrate through the reaction apparatus. Using the transport sheet,one can avoid difficulties such as rupture in the substrate due to thetake-up tension of the roll and it can be rewound under a reasonablystrong force without considering its strength. An exemplary method ofimplementing this approach is shown in FIG. 7; a roll 7 of substrate inthe form of a long web sheet of woven or non-woven fabric and a roll 7′of transport sheet equal in length to the substrate are joined withinthe nitrogen replacement vessel 1 and transported along the line withinthe reaction apparatus to pass through the graft reaction vessel 5;then, the substrate and the transport sheet are separated from eachother and rewound by the take-up rolls 9 and 9′, respectively.

[0087] Examples of the sheet transporting support material (transportsheet) that can be used for the purpose described above include nets,films, as well as woven and non-woven fabrics that are made fromhalogenated polyolefins, polyethylene terephthalate, polyurethane fiber,vinylon fiber, cellulose fiber and inorganic fibers such as glass fiber.

[0088] If desired, the woven or non-woven fabric material in combinationwith a reinforcement according to the first and second aspects of theinvention or the woven or non-woven fabric material composed of mixedpolymer fibers according to the third aspect of the invention may beused as the long web sheet which is to be subjected to aradiation-induced graft polymerization treatment by the method accordingto the sixth aspect of the invention. Alternatively, in the process ofradiation-induced graft polymerization treatment according to the sixthaspect of the invention, a monomer may be grafted only to the fibersurface according to the fourth and fifth aspects of the invention. Ineither way, the strength of the woven or non-woven fabric material isretained to secure the strength of the fiber itself, thereby solvingproblems such as the above-mentioned rupture in the substrate sheet dueto the tension of the take-up roll. Other problems that can be solvedare those originating from monomer grafting onto the substrate such asthe tendency of the fiber to be easily dislodged from the embrittledsubstrate and the substantial decrease in the size of that substrate.

[0089] Thus, according to its seventh aspect, the present inventionrelates to a method for performing radiation-induced graftpolymerization on substrates in the form of a woven or non-woven fabric,comprising the steps of exposing a substrate in the form of a woven ornon-woven fabric to electron beams in a nitrogen atmosphere, contactingthe irradiated substrate with a specified amount of monomer in anitrogen atmosphere, and subjecting the monomer and the substrate inmutual contact to graft polymerization in a nitrogen atmosphere, saidfirst through third steps being performed in succession, characterizedin that the substrate in the form of a woven or non-woven fabric is apolymer substrate for radiation-induced graft polymerization in the formof a woven or non-woven fabric that comprises a woven or non-wovenfabric composed of polymer fiber and a reinforcement polymer having agreater strength and a slower rate of radiation-induced graftpolymerization than said polymer fiber or a polymer substrate forradiation-induced graft polymerization in the form of a woven ornon-woven fabric which is formed by mixing polymer fiber with anotherpolymer fiber having a slower rate of radiation-induced graftpolymerization than said polymer fiber. Further, according to its eighthaspect, the present invention relates to a method for performingradiation-induced graft polymerization on substrates in the form of awoven or non-woven fabric, comprising the steps of exposing a substratein the form of a woven or non-woven fabric composed of polymer fiber toelectron beams in a nitrogen atmosphere, contacting the irradiatedsubstrate with a specified amount of monomer in a nitrogen atmosphere,and subjecting the monomer and the substrate in mutual contact to graftpolymerization in a nitrogen atmosphere, said first through third stepsbeing performed in succession, characterized in that radiation-inducedgraft polymerization reaction is performed under such conditions thatonly the surface of said polymer fiber undergoes radiation-induced graftpolymerization whereas no monomer is grafted onto the center of thepolymer fiber.

[0090] Industrial Applicability

[0091] According to the present invention, woven or non-woven fabrics ofpolyethylene which have heretofore found only limited use can beemployed to produce substrates for polyethylene graft polymerizationthat show outstanding characteristics.

[0092] In the first and second aspects of the invention, theintroduction of a reinforcement contributes to increasing the strengthof the woven or non-woven fabric itself. What is more, the strength ofthe woven or non-woven fabric will not decrease in any of the steps ofgraft polymerization reaction, ion-exchange group introducing reactionand filter shaping and processing, thereby eliminating the possibilitythat the woven or non-woven fabric material will break or perform poorlyin these steps. If the substrate for graft polymerization according tothe first aspect of the invention is subjected to graft polymerization,undulations occur in the surface of the substrate, making it suitablefor use as filter stock that can be pleated or otherwise shaped intofilters. In the case of pleating, the distance between pleats can beheld constant without using separators that have no adsorbing capabilityon their own but which simply reduce the effective filtration area ofthe filter. In addition, graft polymerization also occurs in thereinforcement to introduce functional radicals there and thiscontributes to a further improvement in the capabilities of the filterassembly. Using no separators, the filter assembly is lighter in weight.The process of filter production is made simple enough to realize costreduction.

[0093] In the fourth aspect of the invention, graft treated polyethylenematerials having improved physical strength and high graft ratio can beproduced using polyethylene fiber in the form of a woven or non-wovenfabric but without using reinforcements. The concept of the inventioncan be applied to a broad range of other polymer materials to producegraft treated polymer materials having improved physical strength andhigh graft ratio.

[0094] The graft treated materials according to the first to fifthaspects of the invention are useful as stock for air cleaning chemicalfilters and ion-exchange filters used in pure water productionequipment.

[0095] In the sixth aspect of the invention, long webs of substrate thathave heretofore been difficult to handle can be subjected to acontinuous radiation-induced graft polymerization treatment, making itpossible to mass-produce long webs of woven or non-woven fabricmaterial. As further advantages, the efficiency of radical utilizationis high enough to reduce monomer consumption and the cleaning step aftergraft polymerization reaction can be eliminated. These combine to reducethe production cost. In addition, the products of graft polymerizationare consistent in quality to have higher yield.

[0096] According to the seventh and eighth aspects of the invention,long webs of substrate can be subjected to a continuousradiation-induced graft polymerization treatment while maintaining theirstrength. This contributes to improving the quality of the products ofgraft polymerization.

[0097] The following examples are provided for further illustrating thepresent invention but are in no way to be taken as limiting.

EXAMPLE 1

[0098] A non-woven fabric was formed of polyethylene fiber having adiameter of 20 μm. It was subjected to embossing to prepare a non-wovenfabric material having an areal density of 60 g/m² and a thickness ofabout 0.35 mm. Polyethylene filaments of 100 d (denier) were knitted asa reinforcement into the non-woven fabric material in a grid patternhaving 2-3 mm openings. The thus prepared substrate in the form of areinforced non-woven fabric had adequate strength and its tensilestrength was 12 kg/5 cm in a longitudinal direction and 5 kg/5 cm in atransverse direction.

[0099] This substrate was exposed to 50 kGy of electron beams andreacted with glycidyl methacrylate for 2 hours to give a graft ratio of146%. Filaments were taken from part of the grafting substrate andmeasured for the graft ratio of the reinforcement filaments, which wasfound to be 37%. Hence, the graft ratio of the non-woven fabric ofpolyethylene fiber was calculated to be about 160%. The appearance ofthe surface of the substrate before and after grafting is shown in FIGS.2 and 3 (micrographs). The thickness of the substrate increased to about1.7 mm since undulations formed on its surface as FIG. 3 shows. Thesubstrate non-woven fabric was sulfonated with a solution of sodiumsulfite to give a filament-reinforced, non-woven fabric of strong acidiccation-exchange fiber. Its neutral salt decomposition capacity measuredto be 2.77 meq/g.

[0100] The tensile strength of the non-woven fabric material aftergrafting was 17 kg/5 cm in a longitudinal direction and 9 kg/5 cm in atransverse direction. Having higher tensile strength than beforegrafting, the non-woven fabric material in no case failed to maintainits shape during introduction of ion-exchange groups, nor did it sufferdislodging of fiber fragments.

[0101] The same non-woven fabric material was used to mold pleatedfilters each having a cross-sectional area of 100 mm square, a depth of50 mm and 10 peaks. No conventional separators were used in filtermolding. Using a gas adsorption test apparatus of the configurationshown in FIG. 8, 5 ppm of ammonia gas was passed through the filters ata rate of 200 L/min. In FIG. 8, numeral 101 refers to a permeator, 102 agas sampling line, 103 a filter mounting section, 104 a suction pump and105 a flow meter. In the initial period, more than 99% of the ammoniasupplied was removed. About 23 hours later, the ammonia removal droppedto 90% when the passage of ammonia gas was stopped. The exchangecapacity consumption was 78%.

Comparative Example 1

[0102] A non-woven fabric was prepared as in Example 1 except thatreinforcement polyethylene filaments were not knitted into the fabric.It was then subjected to radiation-induced graft polymerization andsulfonation reaction as in Example 1.

[0103] Before grafting, the non-woven fabric material had only smalltensile strength values, 0.35 kg/5 cm in a longitudinal direction and0.07 kg/5 cm in a transverse direction. The graft ratio was 131%; afterthe polymerization, the non-woven fabric was interspersed with fiberlumps and varied in thickness between 1 and 5 mm; however, its shape wassubstantially maintained.

[0104] After the sulfonation reaction, the non-woven fabric wasdifficult to handle and in spite of careful handling, it turned tofluffy mass and could not be processed into filters.

EXAMPLE 2

[0105] A non-woven fabric having an areal density of 65 g/m² and athickness of 0.4 mm was formed of polyethylene fiber having a diameterof 10-30 μm. A polyethylene net was thermally fused to both sides of thenon-woven fabric, thereby preparing a substrate for radiation-inducedgraft polymerization. The polyethylene net was prepared by the followingprocedure: a polyethylene film about 0.8 mm thick was torn into yarnswhich were rearranged in a grid pattern having 5 mm openings; further,yarns were stretched diagonally across the grids to form thereinforcement net. The non-woven fabric was then exposed to 50 kGy ofelectron beams and reacted with glycidyl methacrylate for 2 hours togive a graft ratio of 193%. The appearance of the surface of thesubstrate before and after graft polymerization is shown in FIGS. 4 and5 (micrographs). The polyethylene net alone was subjected toradiation-induced graft polymerization under the same conditions andmeasured for its graft ratio, which was found to be 70%. Hence, thegraft ratio of the polyethylene non-woven fabric without the net wascalculated to be 275%. After radiation-induced graft polymerization,undulations formed on the surface of the non-woven fabric material, soits thickness increased to about 4.2 mm. The substrate was thensulfonated with a solution of sodium sulfite to produce anet-reinforced, non-woven fabric material of strong acidiccation-exchange fiber having a neutral salt decomposition capacity of2.98 meq/g.

[0106] Before graft polymerization, the non-woven fabric material hadadequate tensile strength values, 8 kg/5 cm in a longitudinal directionand 7 kg/5 cm in a transverse direction. Even after graft polymerizationand sulfonation reaction, the tensile strength did not drop at all butincreased to 9 kg/5 cm in a longitudinal direction and 15 kg/5 cm in atransverse direction. During the treatments mentioned above, thepolyethylene fiber did not suffer any changes in shape or dislodging offiber fragments.

[0107] Using this non-woven fabric material but without usingconventional separators, pleated filters were molded, each having across-sectional area of 100 mm square, a depth of 50 mm and 7 peaks.Using an experimental setup of the configuration shown in FIG. 8, 2 ppmof ammonia gas was passed through the filters at a rate of 200 L/min.Initially, more than 98% of the ammonia supplied was removed. About 61hours later, the ammonia removal dropped to 90%, when the passage ofammonia gas was stopped. The exchange capacity consumption was 74%.

Comparative Example 2

[0108] A non-woven fabric of polyethylene fiber was prepared as inExample 2 except that no polyethylene nets were thermally fused. Thenon-woven fabric was then subjected to radiation-induced graftpolymerization and sulfonation reaction under the same conditions as inExample 2 to produce a strong acidic, cation-exchange non-woven fabrichaving a neutral salt decomposition capacity of 3.04 meq/g. The tensilestrength of this non-woven fabric material was 6 kg/5 cm in alongitudinal direction and 2 kg/5 cm in a transverse direction; due tocareful handling, the non-woven fabric maintained its shape.

[0109] Using this non-woven fabric material, pleated filters werefabricated and subjected to a gas passage test as in Example 2. However,due to the “failure” of the non-woven fabric, the filters experiencedsuch a great pressure loss that the flow rate of ammonia gas was no morethan 132 L/min. To deal with this difficulty, a corrugated aluminumseparator 5 mm wide was inserted between pleat peaks. This modifiedfilter assembly was subjected to a gas passage test as in Example 2. Thegas removal was 99% in the initial period of gas passage. When 90%removal was reached, the passage of ammonia gas was stopped; theexchange-capacity consumption was 59% and lower than 74%, the valueattained in Example 2.

EXAMPLE 3

[0110] Polyethylene fiber having a diameter of about 20 μm andpolyethylene terephthalate fiber having a diameter of 15 μm were mixedat a weight ratio of 75:25. The mixed fibers were used to form asubstrate in the form of a non-woven fabric having an areal density of45 g/m² and a thickness of 0.25 mm. The tensile strength of thisnon-woven fabric was 8.2 kg/5 cm in a longitudinal direction and 6.7kg/5 cm in a transverse direction.

[0111] A sample of this substrate non-woven fabric (20 cm×20 cm, 1.89 gcontaining 1.42 g of polyethylene) was subjected to a radiation-inducedgraft polymerization treatment as in Example 1 to give a graft productweighing 4.67 g; the graft ratio of glycidyl methacrylate ontopolyethylene was therefore 196%. The non-woven fabric material wassulfonated as in Example 1 to give a strong acidic, cation-exchangenon-woven fabric having a neutral salt decomposition capacity of 2.72meq/g.

[0112] This non-woven fabric measured 19.4 cm long by 20.0 cm wide by0.63 mm thick; with the exception of an increase in thickness, thefabric experienced little dimensional changes in both longitudinal andtransverse directions. Its tensile strength was 7.5 kg/5 cm in alongitudinal direction and 6.1 kg/5 cm in a transverse direction; thedrop in tensile strength was insignificant and the non-woven fabric hada sufficient strength to be processed into filters and to withstandtransport for continuous reaction.

EXAMPLE 4

[0113] Using high-density polyethylene fiber of 6 deniers, a substratein the form of a non-woven fabric having an areal density of 55 g/m² wasprepared by spot welding. The tensile strength of the substrate was 5.3kg/5 cm in a longitudinal direction and 4.1 kg/5 cm in a transversedirection.

[0114] The substrate non-woven fabric was exposed to 150 kGy of electronbeams in a nitrogen atmosphere, impregnated with glycidyl methacrylatein an amount equal to 130% of the weight of the non-woven fabric andsubjected to reaction at 50° C. for 4 hours, giving a glycidylmethacrylate graft ratio of 124%. The thus graft treated substrate wasimmersed in an aqueous solution containing 10% of sodium sulfite and 10%of isopropyl alcohol and sulfonated by reaction at 80° C. for 8 hours,producing a non-woven fabric material of strong acidic, cation-exchangefiber; its neutral salt decomposition capacity measured to be 2.61meq/g.

[0115] The graft treated non-woven fabric material was examined for thedistribution of sulfur atoms in fiber cross section by SEM-XMA (scanningelectron microscope/X-ray microanalyzer). FIG. 9 shows the SEM and XMAphotographs taken for the non-woven fabric material; FIG. 9a is the SEMphotograph of fiber cross sections and FIG. 9b is the XMA photographshowing the distribution of sulfur atoms in fiber cross sections. As isclear from FIG. 9b, sulfur atoms were present in about one third of thefiber radius as measured from the surface, indicating that the fiber hadbeen graft polymerized in the area down to one third from the surfacewhereas about two thirds from the center remained unaffected bygrafting. The tensile strength of the non-woven fabric material in a drystate was 4.6 kg/5 cm in a longitudinal direction and 3.7 kg/5 cm in atransverse direction; the drop in strength was only about 10% of theinitial values and small enough to present no difficulties in thesubsequent processing into filters.

Comparative Example 3

[0116] Using low-density polyethylene fiber of 6 deniers, a substrate inthe form of a non-woven fabric having an areal density of 60 g/m² wasprepared by spot welding. The tensile strength of the substrate was 4.4kg/5 cm in a longitudinal direction and 3.7 kg/5 cm in a transversedirection. This substrate was subjected to radiation-induced graftpolymerization and sulfonated as in Example 1. With the substrate takenas a whole, the glycidyl methacrylate graft ratio was 130% beforesulfonation. The product was a non-woven fabric material of strongacidic, cation-exchange fiber having a neutral salt decompositioncapacity of 2.77 meq/g. Due to inadequate fiber strength, the non-wovenfabric partly failed to maintain its shape and turned to fluffy mass.

[0117] The graft treated non-woven fabric material was examined for thedistribution of sulfur atoms in fiber cross section by SEM-XMA. FIG. 10shows the SEM and XMA photographs taken for the non-woven fabricmaterial; FIG. 10a is the SEM photograph of fiber cross sections andFIG. 10b is the XMA photograph showing the distribution of sulfur atomsin fiber cross sections. As is clear from FIG. 10b, sulfur atoms werepresent throughout fiber cross section down to the center, indicatingthat the graft reaction had progressed to the center of fiber crosssection. Therefore, in Comparative Example 3, graft sites distributedthroughout the cross section of the substrate in sharp contrast withExample 4 in which grafting occurred at higher density in the surface ofthe substrate.

[0118] The tensile strength of the graft treated non-woven fabricmaterial was 1.4 kg/5 cm in a longitudinal direction and 0.3 kg/5 cm ina transverse direction; the drop in tensile strength was so great thatthe non-woven fabric material could not be processed into pleated filterstock without breaking or letting fine fiber scrap occur in a largequantity.

EXAMPLE 5

[0119] A long web sheet (50 cm wide by 600 m long) of a non-woven fabrichaving an areal density of 50 g/m² and a thickness of 0.3 mm was formedof polyethylene (PE) monofilaments having a diameter of about 15 μm andwound into a roll 7. The roll was set in a nitrogen replacement vessel 1(see FIG. 1) and the leading end of the sheet was joined to a dummy net,which extended from a take-up roll 9 past a graft polymerization vessel5, a monomer impregnation vessel 4 and an irradiator 3 such that itstrailing end would reach the nitrogen replacement vessel 1. By rewindingthe dummy sheet, the non-woven fabric sheet 2 would be fed forwardcontinuously.

[0120] Both the nitrogen replacement vessel 1 and the shutter in thewall separating the compartment between the nitrogen replacement vessel1 and the irradiator 3 were closed and the process consisting ofevacuation to 50 mmHg and introduction of nitrogen up to one atmospherewas performed in two cycles to accomplish nitrogen replacement.

[0121] Then, a cooling gas produced by evaporating liquid nitrogen wasintroduced from the compartment so that the temperature in theirradiating section of the irradiator 3 would be set to −20° C. and theoxygen concentration to 200-300 ppm at the outlet of the graftpolymerization vessel 5.

[0122] The conditions for irradiation with the irradiator 3 were set to300 keV and 15 mA and the drive rolls and the sheet rewinder wereadjusted to transport the dummy sheet at a speed of 2 m/min.

[0123] In the next step, the dummy net was gradually taken up so thatthe non-woven fabric sheet 2 joined to the trailing end of the net wascontinuously brought into the irradiator 3 for irradiation, impregnatedwith a solution of glycidyl methacrylate in the monomer impregnationvessel 4 and subjected to graft polymerization reaction in thesubsequent graft polymerization vessel 5 at 50° C. for 20 minutes. Usinga separate non-woven fabric sheet, the squeeze rolls were pre-adjustedto ensure that the irradiated non-woven fabric sheet would have amonomer impregnation ratio of 120%.

[0124] By measuring the areal density of the graft treated non-wovenfabric sheet, the 600-m long sheet in a roll was calculated for graftratio at 100-m intervals and the results are shown in Table 1.Considering the fluctuation in the areal density of the substratenon-woven fabric per se, the graft treated non-woven fabric long websheet obtained in Example 5 was very uniform in graft ratio.

[0125] For the calculation of graft ratio, the product of graftpolymerization was immersed in a solution of dimethylformamide at 60° C.for 6 hours, washed with methanol and dried for weight measurement. Thegraft ratio was determined by the following equation:

Graft ratio=(weight after graft polymerization−weight before graftpolymerization)/(weight before graft polymerization)×100(%)

[0126] Table 1: Graft ratio of 600-m long non-woven fabric long websheet as calculated on the basis of areal density measurement Site ofmeasurement* 0 m 100 m 200 m 300 m 400 m 500 m 600 m Areal density(g/m²) 111 116 119 121 116 122 123 Graft ratio (%) 122 132 138 142 132144 146

Comparative Example 4

[0127] A long web sheet of non-woven fabric material was subjected tograft polymerization under the same conditions as in Example 5 exceptthat the oxygen concentration was set to 1300 ppm at the outlet of thegraft polymerization vessel 5. The graft treated non-woven fabricmaterial as taken up with the sheet rewinder 9 was wet with theunreacted monomer, suggesting that the graft polymerization had notprogressed to the fullest extent.

[0128] Graft ratio measurement was performed as in Example 4 and theresults are shown in Table 2. As the sheet was taken up by about 300 m,there occurred some persistent increase in graft ratio and this wasprobably because the oxygen concentration decreased to some extent onaccount of the nitrogen stream.

[0129] Table 2: Graft ratio of 600-m long non-woven fabric web sheet ascalculated on the basis of areal density measurement Site ofmeasurement* 0 m 100 m 200 m 300 m 400 m 500 m 600 m Areal density(g/m²) 52 53 56 54.5 61 65 70.5 Graft ratio (%) 4 6 12 9 22 30 41

[0130] *The innermost end of the sheet rewound with the take-up roll 9was found 0 m and the outermost end as 600 m.

EXAMPLE 6

[0131] A web sheet of non-woven fabric which was the same as used inExample 5 was treated by the same method as in Example 5, except thatthe oxygen concentration at the outlet of the graft polymerizationvessel 5 was set to 50-100 ppm, the conditions for irradiation with theirradiator 3 were set to 275 keV and 3 mA, and the transport speed ofthe dummy net was adjusted to 1.5 m/min. Not all of the transport rollsin the graft polymerization vessel 5 were used but some of them werebypassed to adjust the time of residence in the graft polymerizationvessel 5 to 20 minutes.

[0132] The result was substantially the same as obtained in Example 5and the graft ratio was high over the whole length of the graft treatedweb sheet in roll form.

EXAMPLE 7

[0133] A web sheet material of non-woven fabric (500 mm wide by 600 mmlong) was prepared from the same substrate in the form of a reinforcednon-woven fabric as used in Example 1 and subjected to radiation-inducedgraft polymerization by the same method as in Example 5. As in Example5, the product of graft polymerization was uniform in that the graftratio was within the range of 135%±15% over the full length of 0-600 mm.

[0134] The initial web had a width of 500±3 mm whereas the width of thegraft treated web was 500±5 mm, causing hardly noticeable dimensionalchanges.

EXAMPLE 8

[0135] A web sheet material of non-woven fabric (500 mm wide by 600 mmlong) was prepared from a substrate non-woven fabric that was formed ofthe same high-density polyethylene fiber as used in Example 4 and it wassubjected to radiation-induced graft polymerization by the same methodas in Example 5.

[0136] As in Example 5, the product of graft polymerization was uniformfor practical purposes in that the graft ratio was within the range of135%±19% over the full length of 0-600 mm.

[0137] The initial web had a width of 500±5 mm whereas the width of thegraft treated web was 500±12 mm, causing hardly noticeable problems forpractical purposes.

Comparative Example 5

[0138] A web sheet material of non-woven fabric (500 mm wide by 600 mmlong) was prepared from a substrate non-woven fabric that was formed ofthe same low-density polyethylene fiber as used in Comparative Example 3and it was subjected to radiation-induced graft polymerization by thesame method as in Example 5.

[0139] As soon as the reaction started, the web sheet shrank to a widthof 400 m and less, tearing apart when a length of 43 m was unwound. Thesheet did not have high enough tensile strength in a longitudinaldirection to withstand reaction in web form.

[0140] The present invention can be implemented in the following variousembodiments.

[0141] 1. A polymer substrate for radiation-induced graft polymerizationin the form of a woven or non-woven fabric that comprises a woven ornon-woven fabric composed of polymer fiber and a reinforcement polymerhaving a greater strength and a slower rate of radiation-induced graftpolymerization than said polymer fiber.

[0142] 2. A polymer substrate for radiation-induced graft polymerizationin the form of a woven or non-woven fabric that comprises a woven ornon-woven fabric composed of polyethylene fiber and a reinforcementpolyethylene having a greater strength and a slower rate ofradiation-induced graft polymerization than said polyethylene fiber.

[0143] 3. The polymer substrate for radiation-induced graftpolymerization as set forth in the preceding paragraph 1 or 2, whereinsaid reinforcement is in the form of filaments which are knitted intothe woven or non-woven fabric.

[0144] 4. The polymer substrate for radiation-induced graftpolymerization as set forth in the preceding paragraph 1 or 2, whereinsaid reinforcement is in the form of a net which is joined to thesurface of the woven or non-woven fabric.

[0145] 5. A polymer substrate for radiation-induced graft polymerizationin the form of a woven or non-woven fabric which is formed by mixingpolymer fiber with another polymer fiber having a slower rate ofradiation-induced graft polymerization than said polymer fiber.

[0146] 6. The polymer substrate for radiation-induced graftpolymerization as set forth in the preceding paragraph 5, wherein saidpolymer fiber is polyethylene fiber and said another polymer ispolyethylene terephthalate fiber.

[0147] 7. Filter stock having undulations on the surface which hasfunctional radicals introduced by radiation-induced graft polymerizationinto the polymer substrate for radiation-induced graft polymerization asset forth in any one of the preceding paragraphs 1-6.

[0148] 8. The filter stock as set forth in the preceding paragraph 7,wherein said functional radicals are ion-exchange groups.

[0149] 9. The filter stock as set forth in the preceding paragraph 8,which has glycidyl methacrylate introduced into the substrate woven ornon-woven fabric by radiation-induced graft polymerization, with saidion-exchange groups having been introduced into saidglycidylmethacrylate.

[0150] 10. A filter fabricated by pleating the filter stock as set forthin any one of the preceding paragraphs 7-9.

[0151] 11. A radiation graft treated material in the form of a woven ornon-woven fabric material that is composed of polymer monofilament fiberof which only the surface has undergone a radiation-induced graftpolymerization but of which the center remains unaffected by grafting.

[0152] 12. A radiation graft treated material which is produced byperforming a radiation-induced graft polymerization treatment on apolymer substrate for radiation-induced graft polymerization in the formof a woven or non-woven fabric that comprises a woven or non-wovenfabric composed of polymer fiber and a reinforcement polymer having agreater strength and a slower rate of radiation-induced graftpolymerization than said polymer fiber, said polymer fiber having beenundergone the radiation-induced graft polymerization only in the surfacethereof but the center of said polymer fiber remaining unaffected bygrafting.

[0153] 13. A radiation graft treated material which is produced byperforming a radiation-induced graft polymerization treatment on apolymer substrate for radiation-induced graft polymerization in the formof a woven or non-woven fabric which is formed by mixing polymer fiberwith another polymer fiber having a slower rate of radiation-inducedgraft polymerization than said polymer fiber, said polymer fiber havingbeen undergone the radiation-induced graft polymerization only in thesurface thereof but the center of said polymer remaining unaffected bygrafting.

[0154] 14. The radiation graft treated material as set forth in any oneof the-preceding paragraphs 1-13, wherein said woven or non-woven fabricmaterial is composed of polyethylene monofilament fiber.

[0155] 15. The radiation graft treated material as set forth in thepreceding paragraph 14, wherein said polyethylene is high-densitypolyethylene.

[0156] 16. The radiation graft treated material as set forth in any oneof the preceding paragraphs 11-15, wherein functional radicals have beenintroduced by radiation-induced graft polymerization.

[0157] 17. Filter stock composed of the radiation graft treated materialas set forth in the preceding paragraph 16, wherein said functionalradicals are ion-exchange groups.

[0158] 18. The filter stock as set forth in the preceding paragraph 17,wherein said functional radicals are selected from among a sulfonegroup, a quaternary ammonium group and a tertiary amino group that havebeen introduced via glycidyl methacrylate.

[0159] 19. The filter stock as set forth in the preceding paragraph 17,wherein said functional radicals are ion-exchange groups that have beenintroduced by grafting sodium styrenesulfonate, vinylbenzyl trimethylammonium chloride or acrylic acid.

[0160] 20. A process for producing the radiation graft treated materialas set forth in any one of the preceding paragraphs 11-16, whichcomprises the steps of forming a woven or non-woven fabric from polymermonofilament fiber and subjecting said woven or non-woven fabric to aradiation-induced graft polymerization treatment under such controlledconditions that only the surface of said fiber undergoes theradiation-induced graft polymerization treatment but that the center ofthe fiber remains unaffected by grafting.

[0161] 21. The process as set forth in the preceding paragraph 20,wherein said radiation-induced graft polymerization treatment isperformed by a pre-irradiation impregnation graft polymerization method.

[0162] 22. A process for producing the filter stock as set forth in thepreceding paragraph 17, which comprises the steps of forming a woven ornon-woven fabric from polymer monofilament fiber, introducing glycidylmethacrylate into said woven or non-woven fabric by subjecting it to aradiation-induced graft polymerization treatment under such conditionsthat only the surface of said fiber undergoes the radiation-inducedgraft polymerization treatment but that the center of the fiber remainsunaffected by grafting, and then sulfonating it by reaction with sodiumsulfite or aminating the same in an aqueous solution of diethanolamine.

[0163] 23. A process for producing the filter stock as set forth in thepreceding paragraph 17, which comprises the steps of forming a woven ornon-woven fabric from polymer monofilament fiber and introducing sodiumstyrenesulfonate, vinylbenzyl trimethyl ammonium chloride or acrylicacid into said woven or non-woven fabric by subjecting it to aradiation-induced graft polymerization treatment under such conditionsthat only the surface of said fiber undergoes the radiation-inducedgraft polymerization treatment but that the center of the fiber remainsunaffected by grafting.

[0164] 24. A method for performing radiation-induced graftpolymerization on substrates in the form of webs of woven or non-wovenfabric, comprising the steps of exposing a substrate in the form of awoven or non-woven fabric to electron beams in a nitrogen atmosphere,contacting the irradiated substrate with a specified amount of monomerin a nitrogen atmosphere, and subjecting the monomer and the substratein mutual contact to graft polymerization in a nitrogen atmosphere, saidfirst through third steps being performed in succession.

[0165] 25. The method for performing radiation-induced graftpolymerization on substrates in the form of webs of woven or non-wovenfabric as set forth in the preceding paragraph 24, wherein the step ofexposing the substrate to electron beams is preceded by a preliminarystep of replacing the air in the substrate with nitrogen gas, saidpreliminary through third steps being performed in succession.

[0166] 26. The method for performing radiation-induced graftpolymerization on substrates in the form of webs of woven or non-wovenfabric as set forth in the preceding paragraph 24 or 25, wherein contactbetween the substrate and the monomer in the second step is performed byfirst immersing the substrate in a solution of the monomer and thendisplacing from the substrate the monomer solution unnecessary for graftpolymerization.

[0167] 27. The method for performing radiation-induced graftpolymerization on substrates in the form of webs of woven or non-wovenfabric as set forth in any one of the preceding paragraphs 24-26,wherein the oxygen concentration in the nitrogen atmosphere in the thirdstep is no more than 1000 ppm.

[0168] 28. The method for performing radiation-induced graftpolymerization on substrates in the form of webs of woven or non-wovenfabric as set forth in any one of the preceding paragraphs 24-27,wherein the conditions for exposure to electron beams are a voltage of100 keV-500 keV, an electron beam current of 3 mA-50 mA and an exposureof 30 kGy-200 kGy.

[0169] 29. The method for performing radiation-induced graftpolymerization on substrates in the form of webs of woven or non-wovenfabric as set forth in any one of the preceding paragraphs 24-28,wherein the temperature of said substrate being exposed to electronbeams in the first step is between −50° C. and +50° C.

[0170] 30. The method for performing radiation-induced graftpolymerization on substrates in the form of webs of woven or non-wovenfabric as set forth in any one of the preceding paragraphs 25-29,wherein the nitrogen replacement in the preliminary step is accomplishedby performing the process consisting of evacuation and introduction ofnitrogen in at least one cycle.

[0171] 31. The method for performing radiation-induced graftpolymerization as set forth in any one of the preceding paragraphs24-30, wherein the woven or non-woven fabric is a polymer substrate forradiation-induced graft polymerization in the form of a woven ornon-woven fabric that comprises a woven or non-woven fabric composed ofpolymer fiber and a reinforcement polymer having a greater strength anda slower rate of radiation-induced graft polymerization than saidpolymer fiber.

[0172] 32. The method for performing radiation-induced graftpolymerization as set forth in any one of the preceding paragraphs24-30, wherein the woven or non-woven fabric is a polymer substrate forradiation-induced graft polymerization in the form of a woven ornon-woven fabric which is formed by mixing polymer fiber with anotherpolymer having a slower rate of radiation-induced graft polymerizationthan said polymer fiber.

[0173] 33. The method for performing radiation-induced graftpolymerization as set forth in any one of the preceding paragraphs24-32, wherein radiation-induced graft polymerization reaction isperformed such that only the surface of the fiber of which the substratewoven or non-woven fabric undergoes radiation-induced graftpolymerization whereas the center of the fiber remains unaffected bygrafting.

[0174] 34. The method for performing radiation-induced graftpolymerization as set forth in any one of the preceding paragraphs24-33, wherein a substrate transporting web material that need not besubjected to a radiation-induced graft polymerization treatment isjoined to the leading end and/or the trailing end of the substrate inthe form of a web of woven or non-woven fabric and transported through areaction apparatus to transport the substrate web of woven or non-wovenfabric through the reaction apparatus and wherein the substrate web ofwoven or non-woven fabric is severed from the substrate transporting webmaterial after the end of the reaction.

[0175] 35. An apparatus for performing a continuous radiation-inducedgraft polymerization treatment on substrates in the form of a web ofwoven or non-woven fabric, which consists of a series arrangement of anelectron beam exposing unit for applying electron beams to a substratein the form of a web of woven or non-woven fabric in a nitrogenatmosphere, a monomer impregnating vessel for bringing the irradiatedsubstrate into contact with a specified amount of a monomer in anitrogen atmosphere and a graft polymerization vessel for grafting themonomer onto the substrate in a nitrogen atmosphere, which apparatusfurther including a transport means by which the substrate web of wovenor non-woven fabric is continuously passed through said units of theapparatus in the order written.

[0176] 36. The apparatus as set forth in the preceding paragraph 35,which has a nitrogen replacement vessel provided upstream of theelectron beam exposing unit for replacing the air in the substrate wovenor non-woven fabric by nitrogen gas.

1. A polymer substrate for radiation-induced graft polymerization in theform of a woven or non-woven fabric that comprises a woven or non-wovenfabric composed of polymer fiber and a reinforcement polymer having agreater strength and a slower rate of radiation-induced graftpolymerization than said polymer fiber.
 2. A polymer substrate forradiation-induced graft polymerization in the form of a woven ornon-woven fabric that comprises a woven or non-woven fabric composed ofpolyethylene fiber and a reinforcement polyethylene having a greaterstrength and a slower rate of radiation-induced graft polymerizationthan said polyethylene fiber.
 3. A polymer substrate forradiation-induced graft polymerization in the form of a woven ornon-woven fabric which is formed by mixing polymer fiber with anotherpolymer fiber having a slower rate of radiation-induced graftpolymerization than said polymer fiber.
 4. Filter stock havingundulations on the surface which has functional radicals introduced byradiation-induced graft polymerization into the polymer substrate forradiation-induced graft polymerization as set forth in any one of claims1-3.
 5. A filter fabricated by pleating the filter stock as set forth inclaim
 4. 6. A radiation graft treated material in the form of a woven ornon-woven fabric material that is composed of polymer monofilament fiberof which only the surface has undergone a radiation-induced graftpolymerization but of which the center remains unaffected by grafting.7. Filter stock composed of the radiation graft treated material as setforth in claim 6, wherein functional radicals have been introduced byradiation-induced graft polymerization.
 8. A process for producing theradiation graft treated material as set forth in claim 6, whichcomprises the steps of forming a woven or non-woven fabric from polymermonofilament fiber and subjecting said woven or non-woven fabric to aradiation-induced graft polymerization treatment under such controlledconditions that only the surface of said fiber undergoes theradiation-induced graft polymerization treatment but that the center ofthe fiber remains unaffected by grafting.
 9. A method for performingradiation-induced graft polymerization on substrates in the form of websof woven or non-woven fabric, comprising the steps of exposing asubstrate woven or non-woven fabric composed of polymer fiber toelectron beams in a nitrogen atmosphere, contacting the irradiatedsubstrate with a specified amount of monomer in a nitrogen atmosphere,and subjecting the monomer and the substrate in mutual contact to graftpolymerization in a nitrogen atmosphere, said first through third stepsbeing performed in succession.
 10. The method for performingradiation-induced graft polymerization on substrates in the form of websof woven or non-woven fabric as set forth in claim 9, wherein the stepof exposing the substrate to electron beams is preceded by a preliminarystep of replacing the air in the substrate with nitrogen gas, saidpreliminary through third steps being performed in succession.
 11. Themethod for performing radiation-induced graft polymerization as setforth in claim 9 or 10, wherein the woven or non-woven fabric is apolymer substrate for radiation-induced graft polymerization in the formof a woven or non-woven fabric that comprises a woven or non-wovenfabric composed of polymer fiber and a reinforcement polymer having agreater strength and a slower rate of radiation-induced graftpolymerization than said polymer fiber.
 12. The method for performingradiation-induced graft polymerization as set forth in claim 9 or 10,wherein the woven or non-woven fabric is a polymer substrate forradiation-induced graft polymerization in the form of a woven ornon-woven fabric which is formed by mixing polymer fiber with anotherpolymer having a slower rate of radiation-induced graft polymerizationthan said polymer fiber.
 13. The method for performing radiation-inducedgraft polymerization as set forth in any one of claims 9-12, whereinradiation-induced graft polymerization reaction is performed such thatonly the surface of the fiber constituting the substrate woven ornon-woven fabric undergoes radiation-induced graft polymerizationwhereas the center of the fiber remains unaffected by grafting.
 14. Themethod for performing radiation-induced graft polymerization as setforth in any one of claims 9-13, wherein a substrate transporting webmaterial that need not be subjected to a radiation-induced graftpolymerization treatment is joined to the leading end and/or thetrailing end of the substrate in the form of a web of woven or non-wovenfabric and transported through a reaction apparatus to transport thesubstrate web of woven or non-woven fabric through the reactionapparatus and wherein the substrate web of woven or non-woven fabric issevered from the substrate transporting web material after the end ofthe reaction.
 15. An apparatus for performing a continuousradiation-induced graft polymerization treatment on substrates in theform of a web of woven or non-woven fabric, which consists of a seriesarrangement of an electron beam exposing unit for applying electronbeams to a substrate in the form of a web of woven or non-woven fabricin a nitrogen atmosphere, a monomer impregnating vessel for bringing theirradiated substrate into contact with a specified amount of a monomerin a nitrogen atmosphere and a graft polymerization vessel for graftingthe monomer onto the substrate in a nitrogen atmosphere, which apparatusfurther including a transport means by which the substrate web of wovenor non-woven fabric is continuously passed through said units of theapparatus in the order written.
 16. The apparatus as set forth in claim15, which has a nitrogen replacement vessel provided upstream of theelectron beam exposing unit for replacing the air in the substrate wovenor non-woven fabric by nitrogen gas.